Who said killed?
Him failing is enough to stain his title.
Also what's the point of garp for the overall plot?
Shanks has a purpose indeed but Garp?
So you think Oda went out of his way to send garp and gathered the sword members just to fail against some commanders?
If Kuzan or BB were there, i could see it.
What does Shiryu or anyone else beating Garp there accomplish for the plot?
Garp is a prime candidate for a BB kill, like shanks.
Garp also has some purpose left. Him being the hero, and actually being against the WG and celestial dragon tyranny plays well into the revolutionary army's topics.
Having Garp be beaten for basically nothing would be a waste of potential.
Also, the event that shocked the world almost certainly isn't happening in Hachinosu imo. It's based on egghead. So the impact of Garp's defeat would be even lower if it were to happen simultaneously with the Egghead incident.